1. Field of the Invention
The present invention relates to an analyzer for analyzing a biological substance and the method thereof.
2. Description of the Related Art
Recent progress in medicine is remarkable and many diseases are rapidly and appropriately diagnosed and treated. Accordingly, people can healthily live mentally and physically. In addition, implementation of a variety of medical inspections such as examination of lifestyle diseases successfully leads to early recognition and treatment. A variety of these diagnoses and inspections are conducted by a clinical analyzer primarily using blood or urine as a specimen (Japanese Patent Application Publication No. Sho 57-82769).
Clinical analyzers employs absorptiometry that analyzes substances such as sugar, proteins, lipids and enzymes in blood being a specimen by use of enzyme reactions or chemical reactions of colorimetric reagents.
In order to decrease a burden on a patient with less invasiveness (reduction of the amount of blood sample) and to lower a cost with less amount of reagent used, clinical analyzers are now further developed to use only a smaller amount of a sample solution to be measured (hereinafter, simply referred to as a sample solution) for analysis. The use of a smaller amount of a sample solution is also beneficial to the reduction of the amount of waste liquids.
However, a smaller amount of sample solution in absorbance measurement cannot be obtained simply by making the analyzer smaller accordingly. Absorbance A follows the Beer-Lambert Law expressed by:
A=εcl
ε: Molar absorption coefficient
c: Specimen concentration
l: Light pathway length
Because of this, when the amount of a sample solution is made small, the light pathway length l needs to be as long as in a conventional analyzer in order to obtain the same level of a change of the absorption coefficient as in the conventional analyzer. Hence, an elongated cell has to be placed in the progress direction of light for reducing the amount of a sample solution, whereby reducing the amount of sample solution simply by miniaturizing the optical system is not realistic. Moreover, when the cross section of a light beam for irradiation is made small in proportion to the reduction of the amount of sample solution, the intensity of a signal obtained by a light detector is decreased, thereby creating the problem of decreasing in measurement precision.
As a measuring device using electrochemical detection, known is an enzyme sensor that uses amperometry as a measuring principle. A glucose sensor, one example of enzyme sensors, uses a hydrogen peroxide electrode. Glucose in blood being a specimen is reacted with dissolved oxygen by the action of glucose oxidase to generate hydrogen peroxide. The generated hydrogen peroxide is converted into electric current by the reaction H2O2→2H++O2+2e− on the hydrogen peroxide electrode, so that the concentration of the glucose is determined by the measurement of the current. In addition, portable clinical analyzers capable of measuring a multinominal substance utilizing the above principle include i-Stat (Clin. Chem. 39/2 (1993) 283-287). In amperometry, the intensity of a signal depends on the area of an electrode, so that making the amount of a reaction solution small is difficult as in absorbance measurement. For example, the amount of electric current generated by redox reaction on the electrode surface of a redox compound is proportional to the product of the concentration of the redox compound and the area of the electrode.
On the other hand, also available is a portable clinical analyzer that measures glucose with use of potentiometry (Japanese Patent Translation Publication No. Hei 9-500727). This sensor includes a working electrode made from gold, platinum or the like and a reference electrode, and uses a sample solution containing an enzyme and a redox compound. Additionally, the working electrode and the reference electrode are connected to a device for measuring a potential difference. When an analyte is added into a sample solution, the analyte is oxidized by enzyme reaction and at the same time the redox compound in an oxidation state is reduced. The potential difference between the working and reference electrodes generated at the time follows the next Nernst equation.
  E  =            E      0        +                  RT                  n          ⁢                                          ⁢          F                    ⁢              ln        ⁡                  (                                    C              ox                        /                          C              red                                )                    
E: Surface potential of working electrode
E0: Standard potential of redox compound
R: Universal gas constant
T: Absolute temperature
n: Difference of charge of oxidation and reduction types of redox compound
F: Faraday constant
Cox: Concentration of oxidation type of redox compound
Cred: Concentration of reduction type of redox compound
The above equation shows that the potential difference between the working and reference electrodes does not depend on the electrode area. As a result, in a portable clinical analyzer using potentiometry, the intensity of a signal does not decrease even if the amount of sample solution is made small.